A frictionally welded, filled thermoplastic article is strengthened by restricting lateral flow of molten material out of the gap between the workpieces, so that a molten pool of substantial depth is maintained from the beginning of melting to the onset of solidification. Each of the panels has at least two layers of lead that are offset one with respect to the other in two directions to define a recessed lip on two adjacent sides of the panel and an overhanging lip on the remaining two sides of the panel. 130 inches, and therefore, a reduced striking face mass.